Had a bit of a play with this last night and it's improved a lot since the last time I used it (when you actually had to sign up to download it for whatever reason).
It's very convenient. You literally just select a game and go. Not having to load a BIOS or even bind your controller buttons is a huge step forward in making Dreamcast emulation (which is fairly difficult to get right for newcomers) more accessible.
Emulation is solid; out of the games I tried I only encountered a handful of minor gameplay bugs and one game breaking bug (although I suspect a bad rip of the game is to blame for this). There are a few graphical glitches here and there but that's to be expected in an emulator this new and it's nothing that really took away from the experience.
However, by far the biggest problem I had with it was the sound emulation. It's awful and it needs major improvements. Across all the games I tested all of them suffered from audio glitches that made the experience ultimately unenjoyable. Sound and music suffered from issues such as playing too loud, not playing at all, stacking up, distortion and not ending when it should. For example in Sonic Adventure's Windy Valley level, the wind ambience would play much louder than it's supposed to and drown out the music and sound effects (the same effect was in all the levels, but I felt that Windy Valley was most noticeable). You can also hear the 'stacking' effect by activating Tikal which plays a sound effect multiple times and distorts the audio. Obviously I expect this to be fixed in subsequent versions and I get that the main priority is getting the emulator stable, but it's a shame as at the current time it's the one thing that stops me from recommending Redream to emulator newcomers who want to try out Dreamcast games.
Also, something about charging for an emulator kind of rubs me the wrong way. I get that developers should be able to be paid for their work but offering a 'premium' version and paywalling certain features just feels sketchy. I'd much rather the developer just adds a 'pay-what-you-want'/donation button instead.
All in all though, I'm really impressed. Some hard work has obviously gone into this and I can't wait until they sort out the sound issues.